New multi-media techniques such as Voice over IP (VoIP) and Internet Protocol Television (IPTV) open a whole range of new multi-media services. One type of these services enable a group of users to separately watch the same TV channel and communicate with each other using text, audio and/or video. Such services require that the output signal of the end-terminals is transmitted at the same time to all users in the group. In other words, the outputs of the display devices in the group, e.g. televisions, PDAs, mobile devices, PCs or a combination thereof, should be synchronized.
In an IPTV system the TV channel signal is typically transmitted as one or more packetized streams over an high-bandwidth IP network of an operator via network nodes such as head ends, edge routers and access nodes to the end-terminals of the subscribers to such services. During transmission of the streams, the packets are subjected to unknown delays in the network such as transmission delays, differences in network routes and differences in coding and decoding delays. As a consequence the temporal relationship between packets of audio and video streams received at one end-terminal and those received at another end-terminal will be disturbed.
To stream the IPTV content to the end-terminals usually the Real-Time Transport Protocol (RTP) is used. RTP provides sequence numbering and time stamping. Using RTP the temporal relation in one stream (intra-stream synchronization) and between associated streams (inter-stream synchronization) can be restored.
In order to achieve group-synchronization or inter-destination synchronization (as required by the services as referred to above) further measures are necessary. Several techniques are known and all employ time-stamping and variable-delay buffers in the end-terminals. A variable-delay buffer is capable of delaying a stream for a certain amount of time.
In the article “An Application-Level QoS Comparison of Inter-Destination Synchronization Schemes for Continuous Media Multicasting” by Nunome et al. (IEICE trans. Commun. vol. 87, 2004, pp. 3057-3067) two types of inter-destination synchronization schemes are described. A first type uses a central synchronization master, which collects timing information from all end-terminals in the group and adjusts the output timing by distributing control packets to the end-terminals. A second type regards a distributed control scheme, wherein each end-terminal multicasts all timing information to all other end-terminals in the group. Implementation of such schemes in a typical IPTV system introduces however certain problems.
A first problem relates to the limited scalability of the proposed schemes. A central synchronization master is only capable of handling a limited number of end-terminals. Further, a distributed scheme requires a multitude of multicast channels, which is a limited resource.
A second problem relates to the use of controllable variable-delay buffers in the end-terminals. These buffers and the additional functionality for receiving and transmitting timing information imply increased costs of the end-terminal and the inability to use legacy terminals that do not have these capabilities. Moreover, if a user of an end-terminal joins a synchronized group, the local buffering at the end-terminals takes a significant adaptation time before the user is in synchronization with the other end-terminals in the group.